Which came first, the chicken or the egg?
At first glance, this seems like a reasonable question. But most questions have hidden assumptions, and this question has tons of them. And as it turns out, most of the assumptions are incorrect – meaning that the question – as it is usually asked or understood – is actually meaningless.
The question assumes that (a) chickens and eggs have existed continuously, without change, for a long period of time (b) that chickens (vaguely defined!) lay eggs (also vaguely defined!), and (c) that eggs hatch into chickens.
Problem? None of these assumptions are true. They only appear to be true because people only look at chickens and eggs over a very short period of time (perhaps weeks, a year, or when reading books, thinking back over the last 5000 years.)
But birds and their ancestors have been continuously changing for millions of years – and so has the way that their ancestors reproduced. The first chickens… may not even have been chickens, but rather some other form of bird that no longer exists. And those earlier birds are descendants of a branch of the dinosaur family tree; and those early dinosaurs are a branch of the reptile family tree. And over very long, deep periods of time, the way that these organisms reproduced has actually changed!
In fact, the first eggs developed millions of years before anything we even know as birds existed.
FIGURE 7. Simplified phylogeny showing hypothesized stages in the evolution of reproductive traits toward modern birds. Exact locations of stages 1 and 4 are unclear, given the complex distribution of traits in basal theropods and the lack of information for basal Aves and Ornithuromorpha.
Synapomorphies: Stage 1, pre-maniraptoran theropods—bilaminar eggshell with a mammillary and second layer composed of narrow shell units, irregularly distributed squamatic.
Stage 2, oviraptor-grade maniraptorans – increase in relative egg size, more elongate egg shape, slight asymmetry, monoautochronic ovulation, iterative laying, eggshell with more pronounced continuous layer and well-developed squamatic ultrastructure, prominent surface ornamentation, large and highly organized clutches, incubation involving nearly full burial with attendant adult, possibly paternal care.
Stage 3, troodontidgrade paravians—loss of surface ornamentation, increasing asymmetry, low porosity, potential for third (external) layer in eggshell, clutches of ‘‘planted’’ and near vertical eggs, improved contact incubation with tighter clutch configuration, and exposed upper portions of eggs.
Stage 4, Enantiornithes—loss of function in right ovary and oviduct, increasing relative egg size, reduction in egg elongation, incubation as in troodontids or as singleton eggs fully buried in sandstone.
Stage 5, basal Neornithes—eggs show further increase in relative size, more variable and less elongate egg shape, clutch free of sediment cover, egg rotation, chalazae with potentially greater incubation efficiency.
Source: Reproduction in Mesozoic birds and evolution of the modern avian
reproductive mode. Authors – David J. Varricchio and Frankie D. Jackson
The Auk: Ornithological Advances Volume 133 p.654–684, 2016 American Ornithologists’ Union